The present invention relates to a polymer electrolyte fuel cell to be used for a portable power source, an electric vehicle, a cogeneration system, and so on.
It is a fundamental principle, on which a polymer electrolyte fuel cell is based, that a fuel gas supplied to the anode side of an electrolyte membrane-electrode assembly (MEA hereafter) electrochemically reacts with an oxidant gas supplied to the cathode side of the MEA through the electrolyte membrane so as to produce water, whereby an electric energy and a thermal energy are simultaneously generated, the electric energy being used depending on uses and needs.
A representative structure of such fuel cell is shown in FIG. 1, wherein a lower half thereof is a front view and an upper half is mainly a cross-sectional view.
Referring to FIG. 1, MEA 10 comprises a polymer electrolyte membrane 11 and two electrodes, i.e. cathode 12 and anode 13, sandwiching the membrane 11. At outer peripheries of the cathode and the anode, gaskets 14 and 15 are respectively arranged so as to prevent the supplied fuel gas and oxidant gas from leaking to outside and from mixing with each other.
A basic unit of a fuel cell, namely unit cell, is such a structure that an MEA is sandwiched by an anode side separator plate having a gas flow channel to supply and exhaust the fuel gas to and from the anode, and by a cathode side separator plate having a gas flow channel to supply and exhaust the oxidant gas to and from the cathode.
A stacked fuel cell is one made by stacking several tens to several hundreds of such unit cells provided with a cooling unit for every 2 to 3 unit cells, which is called cell stack. In FIG. 1, four kinds of separator plates are used, and only four unit cells are schematically shown for simplifying the drawing. A cathode side separator plate 22 placed at the leftmost end of the cell stack 16 has an oxidant gas flow channel 32, while an anode side separator plate 21 placed at the rightmost end of the cell stack 16 has a fuel gas flow channel 31. Each of separator plates 20 placed among MEAs has an oxidant gas flow channel 34 on a surface thereof facing the cathode, and also has a fuel gas flow channel 33 on a surface thereof facing the anode, so that each separator plate 20 functions both as a cathode side separator plate and an anode side separator plate. A cooling unit comprises a composite separator plate made by combining an anode side separator plate 23 and a cathode side separator plate 24. The cathode side separator plate 24 has an oxidant gas flow channel 36 on a surface thereof facing the cathode, and also has a cooling water flow channel 38 on an opposite surface thereof. The anode side separator plate 23 has a fuel gas flow channel 35 on a surface thereof facing the anode, and also has a cooling water flow channel 37 on an opposite surface thereof. By joining the separator plates 23 and 24 in a manner that the cooling water flow channels thereof face each other, one composite cooling water flow channel is formed by the flow channels 37 and 38.
On each of the both ends of the cell stack 16, a current collecting plate 6, an insulating plate and an end plate are stacked in this order. They are tightened by bolts 70 penetrating therethrough and nuts 71, and are supplied with a tightening pressure by use of washers 73.
In this stacked fuel cell, the end plates, the insulating plates, the current collecting plates and the MEAs have common inlet side manifold holes and common outlet side manifold holes. The reactive gases and the cooling water are supplied to the respective separator plates through the inlet side manifold holes, and are exhausted through the outlet side manifold holes. With reference to FIG. 1, an inlet side manifold hole 18a for oxidant gas in the cell stack 16 is shown. FIG. 1 also shows a manifold hole 1a provided at one end plate 4, and an inlet pipe 2a having an end thereof welded to an edge of the manifold hole 1a. The oxidant gas introduced from the pipe 2a flows through the manifold holes provided at the insulating plate, the current collecting plate and the inlet side manifold hole 18a of the cell stack 16, and flows into the oxidant gas flow channels of the respective cathode side separator plates for reaction, wherein an excessive oxidant gas and products produced by the reaction are exhausted out of an oxidant gas outlet pipe 2b provided at the other end plate through outlet side manifold holes. Similarly, the fuel gas is introduced into an introduction pipe 3a welded to one end plate 4, and flows through fuel gas inlet side manifold holes, fuel gas flow channels of the separators and outlet side manifold holes, and is then exhausted out of a fuel gas outlet pipe 3b. 
Each current collecting plate 6 is a metal plate for collecting the electric power from the serially stacked cell stack and for connecting the same to the outside. Usually, the current collecting plate is made of stainless steel, cupper, brass or the like, and is often provided with a coating such as plated gold for the purpose of decreasing the contact resistance and increasing the corrosion resistance. Each insulating plate 5 is a resin plate for electrically insulating the end plate 4 and the current collecting plate 6. Each of the end plates 4 is a tightening plate for evenly applying a tightening pressure to the cell stack, and is usually made of a machined stainless steel, wherein pipes for introducing and exhausting the reactive gases and the cooling water are welded to the end plates. Further, for securing sealing among above described elements, they usually have grooves for receiving O-rings at peripheral portions around the manifold holes, whereby the O-rings placed in the grooves function the sealing. In FIG. 1, O-rings 8a, 8b and 28 and those without reference numerals are shown.
According to conventional fuel cells, usually a tightening pressure of about 10.0 to 20.0 kgf/cm2 is used for tightening the cell stack in order to decrease the contact resistance among the electrolyte membranes, electrodes and separators and to secure the gas sealing properties of the gaskets. Therefore, the end plates are generally made of metal materials having high mechanical strengths, wherein the cell stack is tightened by applying a tightening pressure to the end plates at both ends thereof, using a combination of tightening bolts and springs or washers. Further, since the supplied humidified gases and the cooling water touch portions of the end plates, usually stainless steel materials, which have high corrosion resistances, are selected from among metal materials and used for the end plates in order to avoid corrosions by such gases and water. The current collecting plates are usually made of metal materials having higher electric conductivities than those of carbon materials, and are in some cases subjected to surface treatment for lowering contact resistances. Furthermore, the end plates at the both ends of the cell stack are electrically connected to each other by the tightening bolts, the insulating plates having electrically insulating properties are each inserted between the current collecting plate and the end plate for securing insulation between them.
The separator plates to be used for such polymer electrolyte fuel cell need to have high electric conductivity, high gas tightness to the reactive gases, and high corrosion resistance to the reaction during oxidization and reduction of hydrogen and oxygen, namely high acid resistance. For these reasons, conventional separator plates in some cases are made of carbon plates having high gas-impermeabilities, with gas flow channels being made by cutting the surfaces of the carbon plates, or in other cases are each made by pressing a mixture of a graphite powder and a binder with a pressing mold having a configuration for forming gas flow channels, and by firing the same.
Recently, metal plates such as stainless steel are attempted to be used for the separator plates in place of the conventionally used carbon materials. The separator plates using metal plates are likely to get corroded or dissolved during a long period use, because the metal plates are exposed to acid atmosphere at high temperatures. When the metal plate gets corroded, the electric resistance of the corroded portions increases, so that the cell output decreases. Further, when the metal plate gets dissolved, the dissolved metal ions are diffused to the polymer electrolyte and are trapped by ion exchange sites of the polymer electrolyte, whereby consequently the ionic conductivity of the polymer electrolyte per se decreases. It is an ordinary way, therefore, to plate gold to have some thickness on the surface of the metal plate for the purpose of avoiding above described deterioration of ionic conductivity.
As described above, stainless steel plates are usually used for the end plates from the viewpoint of mechanical strength. However, in such case, a relatively thick stainless steel material of about 15 mm or thicker needs to be used therefor, because relatively high tightening pressure needs to be applied to the cell stack, thereby to cause a heavy weight of the resultant fuel cell.
Further, since a thick stainless steel cannot be processed by inexpensive molding processes such as die casting and sheet metal processing, cutting work is needed for the end plates. For starting the power generation of a fuel cell, it is usually necessary to firstly increase the temperature of the fuel cell to a given cell temperature. However, when metal plates such as stainless steel are used for the end plates, a problem arises in that it takes a longer time to start the power generation, because metal materials have higher thermal capacities than those of e.g. resin materials. Furthermore, metal materials are likely to quickly radiate heat. Therefore, when end plates are made of metal plates, it is necessary to provide sufficient heat insulating materials thereto for preventing heat radiation.
In addition, the end plates need to be provided with supply inlets and exhaustion outlets for gases and cooling water. For such purpose, it is necessary according to conventional way either to weld a tube-shaped stainless steel material is to the end plate, or to provide the end plate with a part receiving means such as a screw hole and fittedly join a piping part to the part receiving means. Furthermore, insulating plates are indispensable for the conventional end plates made of electrically conductive materials. Besides, in order to stack a combination of a current collecting plate, an insulating plate and an end plate which are made of different materials, it is necessary to use sealing materials such as O-rings for sealing the gases and the cooling water.
It is an object of the present invention to provide a polymer electrolyte fuel cell, in which one or more of above described problems have been solved.
More specifically, it is an object of the present invention to provide a polymer electrolyte fuel cell, in which an insulating plate between an end plate and a current collecting plate becomes unnecessary.
It is another object of the present invention to provide a polymer electrolyte fuel cell, in which the fuel cell is inexpensive and light in weight, and/or efficient in the utilization of thermal energy, and/or high in the corrosion resistance.
A polymer electrolyte fuel cell according to the present invention comprises: a cell stack comprising plural electrically conductive separator plates and electrolyte membrane-electrode assemblies respectively sandwiched between neighboring ones of the separator plates, each of the electrolyte membrane-electrode assemblies comprising a pair of electrodes and a polymer electrolyte membrane sandwiched between the pair of electrodes; a pair of current collecting plates sandwiching the cell stack; a pair of end plates sandwiching the cell stack provided with the pair of current collecting plates; a tightening means for tightening the pair of end plates so as to apply a tightening pressure to the cell stack; gas supply and exhaustion means for supplying, to the cell stack, and exhausting, from the cell stack, an oxidant gas and a fuel gas, the gas supply and exhaustion means comprising an oxidant gas inlet, an oxidant gas outlet, a fuel gas inlet and a fuel gas outlet, and also comprising an oxidant gas flow channel for connecting the oxidant gas inlet and the oxidant gas outlet and a fuel gas flow channel for connecting the fuel gas inlet and the fuel gas outlet, wherein each of the pair of end plates is made of electrically insulating resin-dominant material comprising resin as a main ingredient. The term xe2x80x9cresin-dominant materialxe2x80x9d used herein means a material having resin as a main ingredient, which may contain a filler or reinforcing material such as glass fiber and ceramic powder in case of need.
According to polymer electrolyte fuel cell of the present invention, the end plates are made of a resin-dominant material in place of a conventional metal material, so that the cost and weight of the fuel cell can be very much reduced, because e.g. conventionally needed insulating plates can be omitted. Further, since the resin-dominant material is slower in its heat radiation than metal materials, so that it is superior in utilizing thermal energy. Further, since it becomes possible to remove, in the fuel cell, portions where the gases and the cooling water contact metal materials, so that the corrosion resistance of the fuel cell can be very much improved.
The end plates are each preferred to comprise an injection-molded body made of the resin-dominant material.
Each of the current collecting plates and each of the end plates are preferred to constitute an integrally molded body, wherein the current collecting plate is fittedly embedded in the end plate.
Each of the gas inlets and the gas outlets is preferred to have a shape of cylinder, and to be structured to protrude from a main surface of each of the end plates.
Alternatively, each of the gas inlets and the gas outlets is preferred to have a shape of cylinder, and to be structured to protrude from an end surface of each of the end plates.
The resin-dominant material of the end plates is preferred to contain a reinforcing material such as glass fiber, and the resin of the resin-dominant material is preferred to be selected from polyphenylene sulfide, liquid crystal polymer and polysulfone.
The tightening pressure by the tightening means is preferred to be from 1.5 to 5.0 kgf/cm2 per unit area.
Each of the end plates is preferred to further have a reinforcing member provided on an outer main surface thereof.
While the novel features of the present invention are set forth particularly in the appended claims, the present invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.